The fundamental question in attempts to understand differentiation processes involves the elucidation of the mechanisms controlling the differential expression of the genes within each cell. This control is manifested at many levels, including the synthesis and degradation of given enzymes at specific times and places within the various cells and tissues of developing higher organisms. To this end, we have developed a model assay system in maize for studying the regulation of gene expression, and have made progress in characterizing the various mechanisms involved in this regulation. The model system involves the genetic, biochemical and physiological regulation of the enzyme catalase in maize. A number of regulatory mechanisms appear to contribute to the overall pattern of catalase expression. The qualitative and quantitative activity profile of catalase during seed development and sporophytic differentiation is determined, at least in part by the differential expression of at least two structural genes, changing rates of synthesis and degradation, changing patterns of subcellular compartmentation, and by an endogenous proteinaceous catalase-specific inhibitor. We are currently studying this system in an effort to understand the underlying molecular mechanisms, which we feel will have far-reaching implications for developmental genetics and biology in general.